Yes, of course noone thinks about this kind of thing constantly, but that's not really the point that >>11 was trying to make, it's that people all have weird things they do or think about, regardless of how "normal" they may seem...
(And of course, you don't want to be thinking about the fact that the girl you're kissing may have had her last BF cum on the very lips you're kissing right now, but I'm sure the thought does cross your mind at some point, just where else those lips you're kissing have been...)